This invention relates to the production of high purity F.sub.1 hybrid seeds in those species of the genus Brassica which exhibit strong self-incompatibility, and more particularly concerns a method of producing high purity F.sub.1 hybrid Brassica seeds in high yield without the prior necessity of labor-intensive bud pollination to produce nearly homozygous, inbred parent plants.
The current procedures used to produce such F.sub.1 hybrid Brassica seeds are widely recognized as having serious limitations, both in terms of cost and as regards seed purity. They all require the establishment and provision of stable, sib-incompatible and self-incompatible, nearly homozygous, parental breeding lines, which are available after repeatedly selfing to generate inbred lines. Because Brassica hybrid seed production systems based on the self-incompatible traits of Brassica must utilize strongly self-incompatible plants (an individual plant which will not readily pollinate itself), inbreeding to develop and maintain the parental lines can only be effected by labor intensive techniques, such as bud pollination.
These difficulties in developing and maintaining the parental inbred breeding lines are reflected not only in high hybrid seed cost as a result of the labor cost, but, in practical effect, have required that the breeding lines be maintained in foreign, low-labor-cost countries.
Moreover, the parental lines are of necessity highly inbred, and in Brassica the plants have low vigor, resulting in low F.sub.1 hybrid seed yields.
When making hybrid seed from two parental lines, one of three approaches is used. In the first approach, the flowers in one line are emasculated by hand, and are fertilized with pollen from the crossing line. This manifestly is labor intensive, and consequently expensive. A second approach, widely used on a commercial scale, avoids emasculation. Two inbred parental lines, where one or both lines is sib-incompatible (does not readily pollinate with its siblings) as well as self-incompatible (nearly incapable of fertilizing itself) are crossed by natural pollination. Hybrid seed is collected only from the sib-incompatible breeding line. In the usual case, where one inbred is sib-incompatible, a high ratio, typically 3:1, of the sib-incompatible inbred to the normal inbred is used, and only the seed produced on the sib-incompatible inbred is harvested for sale. Ideally, if both inbreds are sib-incompatible, a 1:1 ratio of the two can be used, and, due to the reciprocity rule of genetics, the entire seed crop can be harvested.
A third approach, also widely used commercially, involves the use of male sterility in the maternal seed parent to prevent self-pollination. Hybrid seed is thus collected from the maternal seed parent. However, utilization of self-incompatibility is superior to that of male sterilty in view of the F.sub.1 seed yield since the F.sub.1 seed can be obtained from both self-incompatible parental lines.
While the sib-incompatible technique for F.sub.1 hybrid Brassica seed production avoids many of the difficulties of emasculation, a number of problems are encountered in practice. One such problem is that large populations of inbreds are used, and these tend to drift genetically due to their being sexually reproduced.
An especially serious seed quality problem arises with the use of sib-incompatible hybrid techniques. If a "misnick" (poor timing between the two inbreds in their going into inflorescence) occurs, some sibbing takes place on the ostensibly sib-incompatible parent, giving rise to inbreds being present in the seed crop. This may result in the seed not being of sufficiently high purity to comply with present day labeling laws, which mandate that a hybrid seed contain at least 95% of the designated hybrid.
And finally, the development of a sib-incompatible inbred normally requires at least about 10 years of inbreeding.
It is known in the art to vegetatively propagate various Brassicas, including self-incompatible Brassicas, and further, to maintain such inbred parental lines for purposes of hybrid seed production. The use of cloning to maintain inbred lines overcomes the above-described labor-intensive techniques.